Discovery of a double white dwarf in the Galactic globular cluster NGC 6397

Binaries in the cores of globular clusters are known to prevent the gravitational collapse of the cluster, and simulations predict that the core of NGC 6397 contains a high number of white dwarfs (WDs), of which many are expected to be part of a binary system. In this work, we report the discovery of a compact binary system consisting of two WDs in the centre of the Galactic globular cluster NGC 6397. The system, known in the literature as NF1, was observed as part of a MUSE radial-velocity survey aiming at characterizing the binary population in the centre of NGC 6397. The spectral analysis of NF1 provides an effective temperature of 16 000 K and a surface gravity (log g) of 5.72 (cgs), which is consistent with the characteristics of an extremely low-mass He-core WD. This is further supported by the mass of 0.23 ± 0.03 M<SUB>⊙</SUB> obtained from fitting the star's spectral energy distribution using its HST magnitude in various filters. The system has a circular orbit with a period of 0.54 days. The radial velocities show a large semi-amplitude of 200 km/s, implying a minimum mass of 0.78 M<SUB>⊙</SUB> for the invisible companion, which is likely another WD, or a neutron star if the inclination of the system is smaller than about 50°. Some significant residuals in radial velocity remain with our best orbital solution, and we tested whether a model with a third body can explain these deviations. While this possibility seems promising, additional measurements are needed to confirm whether the star is actually part of a triple system.